1. Field of the Invention
The present invention relates to a hydrogen occlusive alloy, a hydrogen storage film, and a hydrogen storage tank.
2. Description of the Related Art
Hydrogen occlusive alloys are alloys which store hydrogen as an energy source safely and easily. Therefore, hydrogen occlusive alloys have received much attention as a new energy conversion and storage material. The application field of hydrogen occlusive alloys as a new functional material covers a wide range, such as storage and transportation of hydrogen, storage and transportation of heat, conversion of heat-mechanical energy, separation and refinement of hydrogen, separation of hydrogen isotope, cells using hydrogen as an active material, catalysts in synthetics, and temperature sensors. As described above, hydrogen occlusive alloys are applicable to various fields such as mechanical, physical, and chemical fields, and mentioned as one of key materials in future industry.
There are cases where metal elements which exothermically react with hydrogen, that is, which form a stable compound with hydrogen are used alone as a metal which occludes hydrogen, and cases where these metal elements alloyed with other metals are used. Examples of metal elements which can be used alone are Pd, Ti, Zr, V, rare-earth metal elements, and alkaline-earth metal elements.
Alloying these elements produces many advantages. For example, not only occlusive reaction but also release reaction is performed relatively easily, by properly reducing the binding force between metal and hydrogen. Further, occlusion and release properties are improved, such as a magnitude of equilibrium hydrogen pressure (plateau pressure), the size of equilibrium area (plateau area), which are necessary for reaction, and change of equilibrium pressure (evenness) during occlusion of hydrogen. Furthermore, chemical and mechanical stabilities are improved.
Examples of hydrogen occlusive alloys which are known at present are magnesium-based alloys, such as Mg2Ni, MgNi2, Mg—Ni, magnesium-rare-earth alloys (such as La1-XMgXNi2 based alloys). In particular, it has been reported that magnesium-rare-earth alloys occlude a large amount of hydrogen gas.
However, these magnesium-based alloys have a disadvantage that they release very little amount of hydrogen and hardly function as a hydrogen storage material, although they occlude a large amount of hydrogen in gas phase.
U.S. Pat. No. 5,506,069 discloses that electrochemical hydrogen storage materials represented by (base alloy) aMb are amorphous or microcrystalline close to amorphous property, and thereby active regions of the materials increase due to irregularity of the framework. It discloses that a thin film using the electrochemical hydrogen storage material has a high energy density and high hydrogen storage capacity, and that a battery using the thin film has an improved stability of capacity. The base alloy is an alloy of Mg and Ni, and the ratio of Mg to Ni in the base alloy is from about 1:2 to about 2:1. M represents at least one modifier element selected from the group consisting of Co, Mn, Al, Fe, Cu, Mo, W, Cr, V, Ti, Zr, Sn, Th, Si, Zn, Li, Cd, Na, Pb, La, Mm, and Ca. b is greater than 0.5 atomic % and less than 30 atomic %. And the total of a and b is 100 atomic %.
JP-A 2002-302733(KOKAI) discloses a novel hydrogen occlusive alloy which has an excellent hydrogen release property at low temperature not exceeding 200° C. and incurs an inexpensive manufacturing cost. JP-A 2002-302733(KOKAI) discloses 0.01 to 20 mol % of at least one element selected from the group consisting of Li, B, C, Al, Si, Ca, Ti, Cr, Fe, Co, Cu, Zn, Nb, Ag, Sn, Pb, and misch metal elements is added to a hydrogen occlusive alloy containing Mg, Ni, Mn and inevitable impurities and having a composition ratio of Mg3+aNi2+bNi1+c, where a ranges from −1 to 1, b ranges from −1 to 2, and c ranges from −0.45 to 2.
However, both of the electrochemical hydrogen storage material of U.S. Pat. No. 5,506,069 and the hydrogen occlusive alloy of JP-A 2002-302733(KOKAI) only release a little amount of hydrogen, and are not regarded as having an excellent hydrogen release property.
In the meantime, K. Aoki, X. G. Li and T. Matsumoto, Acta Metall Mater., 40, 1717 (1992) discloses that a part of hydrogen occlusive alloys having a C15 type Laves structure becomes amorphous or non-uniform due to occlusion of hydrogen.